Photoconductor comprising cadmium selenide

ABSTRACT

The photoelectric conductor device comprises a first layer of cadmium selenide or cadmium sulfoselenide, a second layer formed on the first layer and made of a substance containing cadmium salt of oxy-acid, and a third layer formed on the second layer, the third layer being made of a high resistance substance having a resistivity of more than 108 ohm-cm and being selected from the group of compounds not including cadmium selenide.

United States Patent [191 Shimizu et al.

[ June 11, 1974 PHOTOCONDUCTOR COMPRISING CADMIUM SELENIDE [75] Inventors: KazuoShimizu; Okio Yoshida, both of Yokohama; Kazuo Terakawa, Yokosuka; Satoshi Aihara, Yokohama, all of Japan [73] Assignee: Tokyo Shibaura Electric Co., Ltd.,

Kawasaki-shi, Japan 22 Filed: Aug. 17, 1971 211 Appl.No.:172,54l

[30] Foreign Application Priority Data 2,843,773 7/1958 Wardley 313/65 A 3,346,755 10/1967 Dresner 3,486,059 12/1969 Kiuchi et al. 313/65 A FOREIGN PATENTS OR APPLICATIONS 1,198,570 7/1970 Great Britain 313/65 A Primary ExaminerRobert Segal Attorney, Agent, or Firm-Kemon, Palmer & Estabrook [57] ABSTRACT The photoelectric conductor device comprises a first layer of cadmium selenide or cadmium sulfoselenide, a second layer formed on the first layer and made of a substance containing cadmium salt of oxy-acid, and a third layer formed on the second layer, the third layer being made of a high resistance substance having a resistivity of more than 10 ohm-cm and being selected from the group of compounds not including cadmium selenide.

2 Claims, 7 Drawing Figures PATENTEDJUII 1 m4 SHEET 2 BF 2 20 4o 6O TARGET VOLTAGE (v) o O m o 40 TARGET VOLTAGE m 0 0 2 PZUWEDQ E40 FIG 5 2 m 5 2 25 .rZmEEDQ vE Q TIME (MINUTE) PHOTOCONDUCTOR COMPRISING CADMIUM SELENIDE This invention relates to a photoelectric conductor device, and more particularly to a photoelectric conductor device containing cadmium selenide.

It is well known in the art that a photoelectric conductor device utilizing cadmium selenide exhibits an extremely high photosensitivity when it is used as the photoelectric target of an image pickup tube. However, the dark current during the operation of such a photoelectric conductor device utilizing cadmium selenide tends to increase relatively rapidly with elapse of the time. Such increase in the dark current degrades the quality of the picture reproduced from the video signal produced by an image pickup tube utilizing such a photoelectric target thus presenting a serious problem.

Accordingly, an object of this invention is to provide an improved photoelectric conductor device having small dark current and which does not vary appreciably with time.

Another object of this invention is to provide a method of manufacturing such an improved photoelectric conductor device having low dark current and especially suitable for use as a photoelectric target of an image pickup tube.

According to one aspect of this invention there is provided a photoelectric conductor device comprising a first layer made of a photoelectric conductive substance containing cadmium selenide, a second layer formed on the first layer and made of a substance containing cadmium salt of oxy-acid, and a third layer formed on the second layer and made of a high resistance compound other than cadmium selenide, said high resistance compound having a resistivity of more than 10 ohm-cm.

According to another aspect of this invention there is provided a method of manufacturing a photoelectric conductor device comprising the steps of vapour depositing a layer of cadmium selenide upon a transparent electroconductive film formed on a face plate under a pressure of from 1 mmHg to 2 X 10 mm Hg; heat treating the layer of cadmium selenide in an inert gas atmosphere for a predetermined time, and then heat treating the layer of cadmium selenide in selenium vapour for a predetermined time; subjecting the layer of cadmium selenide to a heat treatment at a predetermined temperature and then to a quenching treatment in an inert gas atmosphere containing oxygen and selenium vapour whereby to form a layer of cadmium salt of selenious acid; and depositing a layer of a predetermined thickness of a high resistance compound other than cadmium selenide upon the cadmium salt of selenious acid, said high resistance compound having a resistivity of more than ohm-cm.

The invention can be better understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a crosssection of a photoelectric target embodying the invention and utilized in an image pickup tube;

FIG. 2 show a plot of an X-ray diffraction pattern of the photoelectric target shown in FIG. 1;

FIG. 3 is a graph comparing thetarget voltage viz, current characteristics of the photoelectric target and of a prior art target;

FIG. 4 is a graph comparing the target voltage viz, dark current characteristics of the photoelectric target shown in FIG. 1 and of the prior art target;

FIG. 5 is a plot comparing the time viz, dark current characteristics of the photoelectric target shown in FIG. 1 and of the prior art target; and

FIGS. 6 and 7 show modified targets to be used in an image pickup tube of the solid state scanning type.

With reference now to FIG. 1, a transparent electroconductive signal electrode, for example, an N ESA coating 2 is applied onto the inner surface of a transparent glass face plate 1, that is the light receiving surface of an evacuated photoelectric conductor type image pickup tube, a vidicon for example. A first photoelectric conductor layer 3 of cadmium selenide is applied onto the NESA coating 2 to a thickness of about 2 microns. Cadmium selenide may be substituted by a solid solution containing cadmium selenide and cadmium sulfide at a weight ratio of 2 I or by cadmium sulfoselenide. A second or intermediate layer 4 of cadmium salt of oxy-acid, for example a cadmium salt of selenious acid, is applied onto the first photoelectric conductor layer 3. The intermediate layer 4 is coated with a third layer 5 of a high resistance compound other than cadmium selenide and having a high resistivity of more than about 10 ohm-cm, for example zinc sulfide, thus completing a photoelectric target 6. The intermediate layer 4 may be formed of a mixture of cadmium salt of selenious acid and cadmium oxide instead of a pure cadmium salt of selenious acid. Further, zinc sulfide constituting the third layer 5 may be substituted by a high resistance compound having a resistivity of more than about 10 ohm-cm and selected from the group consisting of germanium sulfide, arsenic disulfide, arsenic trisulfide, arsenic triselenide, germanium selenide, thallium sulfide, thallium selenide, bismuth trisulfide, bismuth triselenide, zinc selenide, cadmium telluride, antimony trisulfide and antimony triselenide.

Although in the foregoing description, the third layer 5 was shown as comprising a single high resistance compound it may be a mixture of two or more high resistance compounds described above or may comprise a multi-layer construction of a single or a mixture of these compounds.

One example of the method of manufacturing the target 6 is as follows:

At first, face plate 1 is coated with a transparent electroconductive film, for example an NESA coating 2, by the well known vacuum deposition technique. The coated face plate is then put into a vacuum of from 1 mmHg to 2 X 10 mm Hg and a layer 3 of cadmium selenide is deposited upon the NESA coating to a thickness of about 2 microns, for example. Prior to the deposition, cadmium selenide may be incorporated with one or more of copper, gold, silver, indium, gallium, aluminium, halogens, tellurium, antimony, bismuth, lead, tin, alkali metals, alkali earth metals and thallium. Further, high purity cadmium selenide of say 99.999 percent may be deposited. Then, the face plate 1 formed with layer 3 in this manner is heat treated for about 15 minutes at a temperature of 600C in an inert gas, for example in nitrogen atmosphere. The face plate is then heat treated for 30 minutes at a temperature of 500C, for example in a vapour of selenium. The purpose of the heat treatment of the cadmium selenide layer 3 in the vapour of selenium is to supply a plurality of deficiences of the selenium in cadmium selenide. in this manner the photoelectric conductor layer 3 of cadmium selenide of high photosensitivity is formed.

The face plate 1 with photoelectric conductor layer 3 formed in this manner is then heated in an inert gas for example nitrogen atmosphere containing oxygen and maintained at a normal pressure, the inert gas containing selenium vapour of the quantity exhibiting a partial pressure of from 1 mm Hg, for example to the partial pressure of the saturated vapour pressure at a predetermined temperature. The heat treated face plate is then cooled rapidly. Thereafter, a second layer 4 of cadmium salt of selenious acid is formed on the first layer 3. Then a third layer of zinc sulfide is vacuum deposited upon the second layer 4 to a thickness of about 0.1 micron, for example, under a vacuum of mm Hg, thus completing a composite target 6.

Although in the foregoing description, the heat treatments for the first and second layers were performed independently, these heat treatments may be combined into a single step. For example, cadmium selenide layer 3 is deposited onto transparent conductive coating 2 formed on the face plate 1 until a thickness of about 2 microns is reached in an inert gas for example nitrogen atmosphere. At this time, face plate 1 is maintained at a temperature of about 150C. The resulting first layer 3 is then heat treated for 10 to 50 minutes at a temperature of 500C in nitrogen atmosphere under normal pressure and containing 0.1 to 10 percent of oxygen. The nitrogen atmosphere is incorporated with selenium vapour of the quantity exhibiting a partial pressure of from 1 mm Hg to the partial pressure of the saturated vapour pressure of the atmosphere at a predetermined temperature. The oxygen in the selenium vapour is effective to decrease the grain size distribution of the fine particles of cadmium selenide in layer 3, thus producing cadmium selenide layer 3 having a substantially uniform grain size. The composition of the atmosphere is adjusted during the cooling step of the target following the heat treatment such that the quantity of oxygen and the partial pressure of the selenium vapour in the treating atmosphere are reduced to zero after a suitable time. In this manner, the second layer 4 of cadmium salt of selenious acid is formed on the first layer 3. Alternatively, the second layer 4 may be formed by heating at a temperature of 300C, for example, the cadmium selenide layer 3 together with selenium dioxide.

Third layer 5 of arsenic trisulfide, for example, is deposited onto the second layer 4 to a thickness of 0.4 micron under a reduced pressure of 10' mm Hg, thus completing composite target 6.

Instead of forming the first layer 3 by the vapour deposition of cadmium selenide, a thin layer of the monocrystal of cadmium selenide having a suitable electrical resistance and photosensitivity may be bonded upon NESA coating 2.

In addition to the cadmium salt of selenious acid, the intermediate layer 4 may be made of a mixture of this cadmium salt and cadmium oxide or a mixture of the cadmium salt of selenious acid and an intermediate compound of cadmium, selenium and oxygen expressed by a molecular formula Cd Se O,,(3CdSeO SeO for example. Where cadmium oxide coexists with cadmium salt of selenious acid, as its resistivity is low, the percentage of cadmium oxide should not be too high. Further, too thick layer of cadmium salt of selenious acid causes a positive after image on the television display screen, thus resulting in the persistence of the image after interruption of the light input. For this reason, a thickness of less than about 2,000A is preferred.

The presence of the layer 4 of cadmium salt of selenious acid or a mixture thereof with cadmium oxide which is formed on the cadmium selenide layer can be affirmed by verifying the result of X-ray diffraction test with the ASTM cards. FlG. 2 shows one example of a portion of such an X-ray diffraction pattern of the photoelectric conductor target which was obtained by using X-rays of CuKa, a type of X-rays in which large peaks of the diffraction show the presence of the hexagonal system of the cadmium selenide in the first layer.

FIG. 3 is a plot to compare the target voltage viz, signal current characteristic of the photoelectric target 6 with a prior art target, in which a solid line curve shows the characteristic of the target whereas a dotted line curve that of the prior art target. In each case, the light input to the target was white light as 2,850K and the brightness at the light receiving surface of the target was 0.5 lux. As can be clearly FIG. 3, from FlG.3, both characteristics are generally similar except that the target operates at a target voltage of few volts higher and that the gamma value of light transfer characteristic of the target is about 0.9, a slight improvement over 0.85 of the conventional design. Under a suitable operating voltage, like the prior art target, the target can also pro duce a signal current of more than 200nA under a brightness of 0.5 lux which shows that the photosensitivity of the photoelectric conductor device is also excellent.

The plot depicted in FIG. 4 compares the target voltage viz, dark current characteristics of the photoelectric target 6 and the prior art target in which a solid line curve shows the characteristic of the target whereas a dotted line that of the prior art target. As FIG. 4 clearly shows, at a preferred target voltage, 40 volts for example, according to the invention it is possible to reduce the dark current to less than lnA and can more effectively suppress the increase in the dark current than the prior target when the target voltage is increased.

FlG. 5 is a plot to compare the dark current characteristics with time of the target and the prior art target. Although the dark current of the prior art target increases rapidly with time as shown by a dotted line, that of the target does not vary appreciably with time as shown by a solid horizontal line.

In this manner, the photoelectric conductor device has a small dark current which is maintained nearly constant over an extended period of use so that when applied to the photoelectric conductor target of an image pickup tube it is possible to greatly improve the signal-to-noise ratio of the television camera, thus improving the operational stability thereof.

In the embodiment illustrated in FIG. 1 third layer 5 is scanned by an electron beam emitted by an electron gun, not shown, to produce from signal electrode 2 a video signal corresponding to the incident light.

FlG. 6 shows a solid state type photoelectric conductor target 6 including a plurality of spaced apart parallel strip electrodes 7 on the outer surface of the third layer 5. In this embodiment, instead of scanning with the electron beam a potential is applied successively to strip electrodes 7 for producing video signals at signal electrode 2.

In another modification shown in FIG. 7, signal electrode is divided into a plurality of discrete strip signal electrodes 2a. This embodiment has better resolution than that shown in FIG. 6.

What we claim is:

l. A photoelectric conductor device comprising a transparent electroconductive film, a first layer formed on said film consisting of photoelectric conductive material selected from the group consisting of cadmium selenide, a mixture of cadmium selenide and cadmium sulfide at a weight ratio of 2: l and cadmium sulfoselenide, a second layer capable of reducing dark current of the device formed on said first layer, said second layer consiwting of material selected from the group consisting of cadmium selenite and a mixture of cadmium selenite and cadmium oxide and having a thickness of less than 2,000A and a third layer formed on said second layer of high resistance compound having a resistivity of more than l0 ohm-cm selected from the group consisting of zinc sulfide, germanium sulfide, arsenic disulfide, arsenic trisulfide, arsenic triselenide, germanium selenide, thallium sulfide, thallium selenide, bismuth trisulfide, bismuth triselenide, zinc selenide, cadmium telluride, antimony trisulfide, antimony triselenide and mixtures thereof.

2. A photoelectric conductor device comprising a transparent electroconductive film, a first layer made of photoelectric conductive material consisting of cadmium selenide, a second layer having a thickness less than 2,000A consisting of cadmium selenite and a third layer formed on said second layer made of a high resistance compound having a resistivity of more than 10 ohm-cm selected from the group consisting of zinc sulfide, germanium sulfide, arsenic disulfide, arsenic trisulfide, arsenic triselenide, germanium selenide, thallium sulfide, thallium selenide, bismuth trisulfide, bismuth triselenide, zinc selenide, cadmium telluride, antimony trisulfide, antimony triselenide and mixtures thereof. 

2. A photoelectric conductor device comprising a transparent electroconductive film, a first layer made of photoelectric conductive material consisting of cadmium selenide, a second layer having a thickness less than 2,000A consisting of cadmium selenite and a third layer formed on said second layer made of a high resistance compound having a resistivity of more than 108 ohm-cm selected from the group consisting of zinc sulfide, germanium sulfide, arsenic disulfide, arsenic trisulfide, arsenic triselenide, germanium selenide, thallium sulfide, thallium selenide, bismuth trisulfide, bismuth triselenide, zinc selenide, cadmium telluride, antimony trisulfide, antimony triselenide and mixtures thereof. 